Thaal Sinestro (/sɪˈnɛstroʊ/) (or simply Sinestro) is a supervillain appearing in American comic books published by DC Comics, particularly those featuring Green Lantern. Created by John Broome and Gil Kane, Sinestro is a former Green Lantern Corps member who was dishonorably discharged for abusing his power. He has since endured as the archenemy of the superhero Green Lantern and is the founder of the Sinestro Corps.

Sinestro has appeared in various forms of non-comics media including shows, films, and video games. Ted Levine, Miguel Ferrer, Xander Berkeley, Ron Perlman, Victor Garber, Jason Isaacs, and others have voiced the character in animated television series and films. The character made his live-action debut in the 2011 film Green Lantern, played by Mark Strong, and will be portrayed by Ulrich Thomsen in the DC Universe, beginning with the series Lanterns.

Sinestro was created by John Broome and Gil Kane, and first appeared in Green Lantern (vol. 2) #7 (August 1961). According to Kane, his appearance was modeled after British actor David Niven.

In 2009, IGN ranked Sinestro as the 15th-greatest comic book villain of all time.

Sinestro is born on the planet Korugar in space sector 1417. His dedication to preserving order originally manifests in his previous career, an anthropologist specializing in the reconstruction of ancient Korugan civilizations. One day, Green Lantern Prohl Gosgotha crash-lands near him, injured and apparently dying. Prohl gives his ring to Sinestro, who barely understands what to do with it before coming under attack from the Lantern's pursuer: a Weaponer of Qward. In the end, Sinestro has to destroy the ruins he was restoring to crush the Qwardian. Afterwards, Gosgotha turns out to still be alive and asks for his ring back so he can summon help. Addicted to the ring's immense power, Sinestro lets him die and convinces the Guardians that the ring chose him as Gosgotha's successor.

In Green Lantern (2005) #45, his wife is shown for the first time in a flashback and revealed to be the sister of Abin Sur.

When Hal Jordan joins the Green Lantern Corps, Sinestro, now a seasoned and respected Lantern, is assigned to be his instructor. Jordan is horrified at his new mentor's totalitarian methods, though Sinestro maintains that he is only doing what he considers necessary to protect Korugar. During his training, Jordan helps Sinestro repel an attempted invasion by the alien warlords known as the Khunds. When Jordan disobeys orders and contacts the Lanterns for help, Sinestro's dictatorship is exposed, and he is forced to appear before the Guardians for punishment. Katma Tui, the leader of a Korugarian resistance movement who feels that Sinestro's "protection" keeps her people from growing as a society through contact with other alien races, is recruited as his replacement in the Corps. Katma Tui eventually grows into one of the most respected Green Lanterns, but she and the rest of Korugar initially resists her appointment to the Corps. Due to Sinestro's actions, Korugar has come to consider the symbol of the Green Lantern Corps an emblem of terror and oppression.

For using the power of the Green Lantern to instill fear rather than combat it, the Guardians banish Sinestro to the antimatter universe, a counterpart to the "real" universe made up of "negative matter". Sinestro ends up on the antimatter world of Qward, that universe's counterpart of the Guardians' homeworld Oa, which is ruled by the Weaponers of Qward, a race of warriors and scientists that bear a fierce hatred of the Guardians and all Green Lanterns. By exiling Sinestro to a world ruled by evil beings who specifically hate Green Lanterns, the Guardians hope to humble him. However, Sinestro is quick to seize the opportunity, persuading the Weaponers that he can use his knowledge of the Corps to destroy them for good.